It is well known that the wear of gun barrel inner surfaces has been exacerbated by the use of propellants with extremely high flame temperatures or in the case where very high energy projectiles are fired in rapid and long burst cycles. This has significantly limited the lifetime of conventional steel gun barrels to unacceptably short times, such that it is widely recognized that higher performance gun barrels are needed.
State-of-the-art gun barrels utilize electroplated chromium as a barrel liner or coating. The thin chromium electroplated coating is cracked and porous as deposited or becomes cracked and porous from the first few projectiles fired through the barrel. The cracked and porous chromium layer permits corrosive propellant gases to attack the underlying steel causing what is termed heat checking which causes the barrel to fail by wear, erosion, corrosion, and excessive fatigue of the steel. Such electroplated chromium steel barrels provide barrel lives of about 20,000 rounds; however, users have expressed a desire to extend barrel lives to 40,000 rounds, or more. In addition, Executive Order D013148 requires the phasing out of hexavalent chromium which is used to deposit chromium coatings on gun barrel bores and other applications.
One solution to improving barrel lifetimes is to substitute a more heat resistant and harder (i.e., more wear resistant) material which suggests either a refractory metal or a ceramic material. An additional requirement is that the liner be applied in a crack- and pore-free state. Due to the high pressures and thermal cycling associated with live firing and the fact that ceramics are inherently brittle, ceramics with fiber reinforcement or ceramic matrix composites (CMC) are preferred over monolithic ceramic materials. The prior art has proposed various gun barrels produced with liners formed of ceramic materials in compression, i.e. the condition in which ceramics are strongest. See, for example U.S. Pat. Nos. 4,401,729 and 5,125,179. Also, U.S. Pat. No. 5,348,598 describes a CMC gun barrel liner formed of a 3-dimensional fiber reinforced ceramic material.
The prior art also has proposed gun barrels with refractory metal coatings, produced usually by sputtering or chemical vapor deposition. See, for example U.S. Pat. Nos. 4,138,512, 4,577,431 and 4,669,212. However, such coatings have an abrupt interface, and are prone to spalling of the protective coating and are inherently brittle due to the physical properties of the deposited refractory metals.
Another limitation to current small caliber steel gun barrels is that they are heavy and cumbersome to carry. Substitution of the steel with a lighter weight metal such as titanium or a light weight metal matrix composite comprised of a lower density material would be a significant advancement for soldiers and law enforcement personnel.
For small caliber barrels in particular, it is extremely difficult to produce rifling in ceramic liners. In addition, such machining tends to fracture the fibers which in turn significantly degrades the mechanical properties.